fix: harden monitor persistence and macOS playback recovery

[tcconnally]

Summary

• Persist the wizard current monitor by stable display identity and restore it across sessions.
• Retry deferred/failed Emby session cleanup after outages without double-counting registry capacity.
• Bound systemic-outage retries and classify FFmpeg Operation timed out messages as transport faults.
• Move reused macOS loadfile operations off the Qt GUI thread with token-checked completion.
• Stop issuing new prefetch work while a systemic outage is active.

Validation

• python3 -m compileall -q hyperwall tests
• python3 tests/run_all.py — overall PASS
• Focused playback contracts: 60/60
• Reliability tests: 73/73
• Remote branch and API read-back match head 5fe08bd13ebcda68e228d19b4ed9155599b7b899.

Evidence boundary

Linux and headless gates pass. Native macOS/Qt/libmpv behavior still requires a target-laptop soak; PyQt6 and the native Emby playback environment are not available in the authoring runtime. Archive-11 analysis and the new regressions are included so that soak evaluates the corrected paths.